The present invention relates generally to interference cancellation in direct sequence spread spectrum systems, and, more particularly, to a novel RAKE receiver that uses spreading code cross-correlations to compute combing weights so as to reduce interference in the RAKE output signals.
Emerging third-generation (3G) wireless communication systems support several different kinds of services including voice, high-speed packet data and multimedia services. Further, 3G systems allow users to access several different services simultaneously. To meet the demand for these services, future wireless communication systems will need to provide much higher capacity than second-generation (2G) systems. Greater capacity can be obtained by allocating additional bandwidth, which is unlikely to occur, or by utilizing the allocated bandwidth more efficiently.
WCDMA (Wideband Code Division Multiple Access) is one technology that is expected to help fulfill the demand for 3G services. WCDMA is a direct sequence, spread spectrum communication system that uses spreading codes to spread narrowband signals over the full width of the frequency channel. Each user transmits over a separate code channel and may transmit simultaneously with other users. Signals from multiple users combine during transmission over the communication channel so that the receiver sees the sum of all users' signals that overlap in time and frequency.
Current implementations of WCDMA use a single-user receiver called a RAKE receiver that separately detects signals from each user without considering other users. The RAKE receiver includes a plurality of RAKE fingers, each of which is matched to a single user's spreading code but aligned with different time delays to detect different multipath echoes of the user's signal. Each RAKE finger includes a correlator that uses the particular spreading code assigned to the user to despread that user's signal. Signals from all other users are treated as noise. A RAKE combining circuit combines the despread signals output from each RAKE finger to obtain a combined signal with an improved signal to noise ratio (SNR).
The conventional RAKE receiver is optimal in white noise. However, time dispersion of the propagation channel results in frequency-selective fading for a wideband signal. As a result, interference caused by other users' signals, i.e., multiple access interference (MAI), and own-signal intersymbol interference (ISI) is colored. MAI is due to cross-correlation between different spreading codes in multipath fading channels. ISI is due to distortion of the transmitted signal that occurs in multipath channels. MAI and ISI limit the capacity of CDMA systems. Also, when colored MAI and ISI are present, the conventional RAKE receiver is not optimal.
Recently, single-antenna Generalized RAKE (GRAKE) receivers have been developed for better suppressing interference. Interference suppression is achieved by treating ISI and MAI as colored Gaussian noise. The noise correlation across fingers is then exploited by adapting the finger delays and combining weights. In this way, the orthogonality between user signals may be partially restored. GRAKE receivers are described in U.S. Pat. No. 6,363,104, and in U.S. patent application Ser. Nos. 09/344,898 and 09/344,899, which are incorporated herein by reference.
In DS-CDMA systems, such as Wideband CDMA and IS-2000, high transmission data rates are achieved by transmitting data at a low spreading factor and/or on more than one spreading code (multi-code). When a low spreading factor and/or multi-code is used, performance is sensitive to multi-path dispersion. With dispersion, there are multiple echoes of the transmitted signal with different relative delays. These echoes interfere with one another. Not only is orthogonality lost between successive symbols as one symbol overlaps with the next, but orthogonality is also lost between symbols sent on different, orthogonal codes.